An inverter, a power conversion apparatus, has found a wide range of applications as a motor speed controller in industrial fields and also with household appliances. Since power semiconductors in the power conversion apparatus generate large losses, the inverter employs a structure that dissipates heat caused by the losses. That is, it has cooling fins and cooling fans so that heat from the power semiconductors as a heat source is transmitted to the cooling fins that in turn exchange heat with air blown to the cooling fins from the cooling fan, thus dissipating heat by an air-cooling method.
In applications where the motor is started and stopped frequently, as with elevators and business washing machines (or coin laundry machines), a so-called power cycle is repeated in which a junction temperature Tj of a chip in a power semiconductor that generates large losses changes from an elevated temperature state (the junction temperature of the silicon chip rises during the operation of the apparatus) to an intermediate temperature state (the junction temperature of the silicon chip lowers while the apparatus is at rest) and to an elevated temperature state (the junction temperature of the silicon chip rises during the operation of the apparatus) again. In such applications a phenomenon is commonly observed in which the power semiconductors fail due to thermal fatigue according to their power cycle longevity.
Power semiconductors are formed of materials with different thermal expansion coefficients (such as silicon chip, copper heat dissipating plate, plastic case and silicon gel), so repetition of thermal expansion and contraction causes thermal stresses in these materials, resulting in disconnection of aluminum bonding wires and cracks in solder, which in turn leads to destruction of the power semiconductors. This phenomenon is called a power cycle.
JP-A-3-261877 discloses a technique that involves estimating a junction temperature of a power semiconductor from an electric current and a temperature of a cooling fin, estimating a degree of fatigue of the power semiconductor using the estimated junction temperature and a power semiconductor operation history calculation means, and displaying a longevity of the power semiconductor when the estimated fatigue value exceeds a predetermined value.
JP-A-7-135731 discloses a technique that involves estimating a junction temperature of a power semiconductor from an electric current, voltage and temperature of a cooling fin, estimating a degree of fatigue of the power semiconductor from the estimated junction temperature, and turning off the power semiconductor when the estimated fatigue value exceeds a predetermined value.
JP-A-8-51768 discloses a technique that involves entering a difference between junction temperatures when an inverter is in operation and at rest, estimating a degree of fatigue of the power semiconductor from a power cycle longevity curve, and turning off the power semiconductor when the estimated fatigue value exceeds a predetermined value.
JP-A-2006-254574 discloses a technique that involves estimating a junction temperature difference of a power semiconductor from an output frequency command value and an output current value and, from a power cycle longevity curve, displaying a remaining percentage of the power cycle longevity of the power semiconductor.
As described above, estimating the junction temperature of a power semiconductor allows the estimation of the degree of fatigue of the power semiconductor and therefore the longevity of power semiconductor parts.